1. Field of the Invention
This invention relates to a method to prevent erosion of the bottom DC electric arc furnace electrodes, while allowing a full tap of the furnace metal heat.
2. Description of the Prior Art
Modern steel production has advanced from the open hearth process requiring from 8 to 20 hours, to more modern processes such as the basic oxygen furnace steel making process which use a lance to blow oxygen into the furnace to produce a heat, where the blowing time is less than 25 minutes. By the term xe2x80x9cheatxe2x80x9d is meant the product of one run. In a basic oxygen furnace, the molten metal product is formed by an initial charge placed in the furnace and comprised of quantities of hot metal, scrap, lime, ore, and spar, and oxygen blown into the furnace at some known rate for a given period of time and from some set lance position. By the term xe2x80x9clancexe2x80x9d is meant the tool (which is in the shape of a lance) with which oxygen is blown into the mass of molten metal within the furnace. The furnace, which is maintained at a high temperature level in the neighborhood of 1200xc2x0 C. to 1650xc2x0 C., processes the charge to produce some quantity of steel, of some analysis and at some end-point temperature, along with some slag, flue gases and losses to thereby complete a heat, as is taught by D. Schroeder et al. in U.S. Patent Specification No. 3,561,743. There, the oxygen content of the molten bath within the basic oxygen furnace (xe2x80x9cBOFxe2x80x9d) was measured by sensors for stack gas analysis and by feedback computation devices, to effect control of positioning the lance oxygen, and to provide heats of specified end-point carbon.
In another, completely different method to make carbon steel, the electric arc process, at least one supported electrode is positioned above a molten metal volume, within the charge materials, in an enclosed furnace containing a molten metal tapping outlet and charging inlets. The charge materials can include chip or granular pig iron, steel scrap, carbon, and lime (xe2x80x9cCaOxe2x80x9d), which are melted by the electrodes at a temperature of about 1600xc2x0 C. to 1700xc2x0 C. produced by an electric arc.
Such electric arc furnaces are described in U.S. Patent Specification No. 3,985,545 (Kinoshita). There, molten metal collects, drop by drop in the bottom of the furnace, after having been melted by the arc and passing through a slag layer which acts as a filter. In the course of the melting reaction carbon monoxide ascends through the molten bath and reacts with oxygen, or oxidizes carbon powder, to form carbon dioxide. The slag layer decarbonizes and desulfurizes the molten steel droplets, which descend through the slag layer to the bottom of the furnace. The slag layer functioned not only as a filter of the drops of molten metal but also as a check or stop for the drops just after they were produced by the arc and filtered. The slag layer was formed to cover the whole space below the lower tip of the electrodes, with the peripheral parts or edge portions of the slag layer turned upward to form a pan-like container. The pan-like slag layer also aided the sliding down of the raw materials in a smooth and sure manner along it to a position below the electrodes.
In U.S. Patent Specification No. 6,024,912 (Wunsche) the charge materials, such as a ferrous scrap mixture, are preheated using heat recovered from emitted hot waste gases from an electric arc furnace. This allows rapid achievement of normal flat bath operating conditions from cold start-up. In U.S. Patent Specification No. 6,238,452 B1 (Kremer et al.) a continuous flow of liquid pig iron melt was fed into an electric furnace along with continuous introduction of refining oxygen gas before the end of charging. This reduces the duration of the melting cycle even though the rate of injection of oxygen is not increased and allows charging without stoppage of heating by the electric arc. The traditional prior art method is described by Kremer et al. as running the electric furnace at maximum power to melt steel scrap (containing residual copper, nickel, and the like) for about 10 minutes, then switching off the electric arc, removing the furnace cover, charging with molten pig iron (typically containing excess 4.5% C and 0.6% Si) for five minutes, then after replacing the cover, switching on the electric arc, resulting in a ten minute shutdown.
In the standard, modern, batch electric arc furnace steel making, each new heat starts with a bottom pool of liquid metal, defined as xe2x80x9cthe heelxe2x80x9d left in the furnace bottom from the previous heat. This served the following purposes: (1) The heel protects the bottom from too rapid an arc bore down without a liquid pool having formed to protect the bottom. When bore-down occurs too rapidly, the arc can go through the refractory bottom; (2) In DC furnaces, the heel is important to protecting the bottom anodes from the arc. If too little heel is present, damage occurs to the anode bottom and the anode bottom can be used for a smaller number of heats. The size of this heel left in the furnace was known to vary in size.
Attempts have been made to measure the depth of heels in DC furnaces so that a sufficient depth of heel could be maintained to protect the anode bottom. The usual practice was to leave more of a heel of product than required, usually 10 wt % to 20 wt % of the previous heat. This meant that from 10 wt % to 20 wt % of the heat was not poured, with a resulting tremendous loss of efficiency. Since the heel left in the furnace is a low-carbon liquid, it would have to be recarbonized by adding carbon, which can take considerable time, and was not completely predictable.
The usage of hot metal starter heels in electric furnaces had been limited mostly to the few integrated steel plants having blast furnaces and electric arc furnaces. The number of DC furnaces in integrated steelworks of the world are also more limited than AC furnaces in these plants. While electric arc carbon steel production provides a lower initial cost as compared with a blast furnace-converter steel manufacturing methods and adjustment of production amounts is easier; there is still a need to increase the production rate and losses associated with retaining a molten heel from the previous run.
In view of this, one of the main objects of the invention is to increase the production rate of electric arc carbon steel production. Another object is to reduce or eliminate the molten heel retained from a previous heat yet still protect bottom DC electrodes in the furnace at the start of the next heat.
The above needs and objects are met by providing a process of operating a DC electric arc furnace in a batch process to produce steel, comprising adding raw iron bearing material, carbon and lime to the furnace, applying current though at least one electrode to provide an arc and supplying oxygen to react and melt the materials to produce molten slag and molten carbon steel; the improvement comprising pouring all the molten carbon steel produced to provide an empty furnace and then adding molten metal to the empty furnace before its next batch operation. The molten metal will preferably comprise pig iron (solid hot metal with a general composition of: C 3.5-4.5%; Mn  less than 1.0%; Si  less than 0.6%; S  less than 0.1%; P  less than 0.3%; with the rest iron). The DC furnace will generally have top and bottom electrodes and the added molten metal will cover at least 100% of the bottom electrodes. This process adds up to utilization of 20 wt % additional molten carbon steel to the initial heat.
The invention also relates to a process of operating a DC electric arc furnace containing top and bottom electrodes, in a batch process to produce molten carbon steel, comprising the steps: (1) providing a furnace empty of molten metal and metal scrap, the furnace comprising a furnace bottom having upward sides and having at least one electrode having a top portion in the furnace bottom, at least one top electrode, an oxygen lance within the furnace, charging openings for raw materials; and exits for slag and molten metal; and then (2) adding molten metal to cover 100% of the furnace bottom electrodes; (3) adding solid raw iron bearing material, carbon and lime; and (4) applying current through the top electrode to provide an arc and supplying oxygen through the oxygen lance to react and heat the raw materials, producing a molten metal layer on top of the furnace bottom and a covering top slag layer, where the reaction generates CO which, along with any carbon, reacts with O2 to form a first rate of CO generation during which CO and CO2 bubble through the slag; and (5) stopping the reaction and pouring out all of the molten carbon steel and molten slag produced at a predetermined molten bath carbon concentration, to provide an empty furnace for the next batch process. The molten metal added in step (2) should preferably be above the bottom electrodes. This method is shown in block diagram form in FIG. 5 of the drawings.
With an initial hot metal heel, early carbon monoxide formation assures the shortest time to form a stable arc for obtaining the highest power input rate. The hot metal heel allows more of the heat to be tapped as product thereby increasing heat size and yield and protects the bottom electrode (anode) at the start of the new batch. The only disadvantage to this process is the time taken to tap out the existing heel and replacing the heel with hot metal. However, this time is more than made up for by the increases in power input rate, due to an earlier stable arc that allows higher power input rates.
The most important iron bearing raw materials used in the process are scrap, DRI (direct residual iron), pig iron, carbon and lime. All of these, can be melted and held in a furnace, preferably a channel induction furnace, associated with the DC electric arc furnace and used to add molten metal as the first part of a new heat.